The present invention pertains generally to data regeneration within high speed optical receivers and the like.
An optical receiver typically includes an optical detector for detecting an incoming optical signal (e.g., bit stream), a linear preamplifier and a decision circuit. The bit stream is comprised of logic zero and logic one data bits transmitted at different signal levels, i.e., "zero" and "one" levels. The decision circuit samples the detected bit stream and determines whether each bit is a one or a zero by comparing its detected signal level to a decision threshold in between the expected zero and one levels. A given sample is assumed to be a logic one if it exceeds the threshold, or logic zero otherwise. In receivers with dynamic adjustment capability, as the optical signal is received, the bit error ratio (BER) of the receiver is measured and the decision threshold is continually adjusted to minimize the BER.
When the amount of uncertainty, or noise, on both one and zero levels is equal, the optimum decision threshold is nominally halfway between the one and the zero level, i.e., in the center of the data "eye". This is usually the case for an AC coupled receiver receiving non-return-to-zero (NRZ) formatted data with a 50% ones density. For this system, the zero level is as far negative from the center of the eye as the one level is positive, and the eye center is at zero volts. A decision level adjustment for this case is generally not necessary.
The situation is different, however, when the noise is asymmetrical. For instance, in avalanche photo-detector (APD) receivers the noise on the ones can be somewhat higher than on the zeros. Nevertheless, a center position for the threshold is usually adequate. With the advent of wavelength division multiplexing (WDM) systems, however, the problem has become much more severe. The optical fiber amplifiers used in these systems exhibit so-called amplified spontaneous emission (ASE) noise, which is dominant on the ones level only. As a result, the optimum threshold is skewed towards the zero level and consequently, an adjustment must be made. To exacerbate the problem, the optical power received on any channel in a WDM link may vary, depending on the number of channels that are active. The reason for such variation is that in an optical amplifier, the total generated power (the sum of the powers of the individual channels) is constant. The optimum threshold will therefore vary as well. A one-time adjustment in this case is inadequate.
In an article by M. Sherif, P. A. Davis, entitled "Decision-point steering in optical fibre communication systems: theory", IEE Proceedings, Vol. 136, Pt.J., No.3, June 1989, analytical expressions are derived and several techniques are discussed for setting and controlling the decision threshold. These techniques are either based on coarse approximations, on extrapolation of measured pseudo bit error rates (PBER) or on analytical expressions. The first approach is inaccurate; the second and third approaches require implementation of search algorithms and numerical computation.
FIG. 1 illustrates a data regeneration circuit 10 that implements decision threshold adjustment using a pseudoerror measurement technique, as disclosed in the Sherif article. An input data signal is applied to both a main decision gate 12 and a pseudoerror decision gate 16. Clock extraction circuit 14 also receives the data signal and derives a clock therefrom to control the sampling instants of the two decision gates. Both decision gates 12, 16 output a solid logic level for each bit, based on a comparison between the bit's signal amplitude and a decision threshold applied to the respective gate. Hence, the output of gate 12 represents regenerated data. Microprocessor 15 generates a variable decision threshold V.sub.TH and a low frequency square wave of a predetermined amplitude. V.sub.TH is applied directly to decision gate 12 as the decision threshold. An adder 13 adds the square wave voltage to V.sub.TH to produce a modified decision threshold voltage which is applied to the pseudoerror decision gate 16. As a result, the bit error ratio of the data output from decision gate 16 is an order of magnitude greater than the BER of the data output by the main decision gate 12, i.e., PBER&gt;BER.
A pseudo error counter 18 counts the pseudoerrors by comparing the outputs of the two gates 12, 16 and generating a count whenever the logic levels in the two paths differ. Microprocessor 15 searches for an optimum value for V.sub.TH by varying V.sub.TH and determining the PBER based on the count for each V.sub.TH. When a minimum PBER is measured, V.sub.TH is assumed to be optimum for that square wave. The square wave amplitude is then changed and the process repeated. An extrapolation is then performed to compute the V.sub.TH value giving a minimum PBER for a zero amplitude square wave. That V.sub.TH value is assumed to be the optimum one, V.sub.OPT. In an alternative method, an analytical expression, rather than an extrapolation, is used to predict the minimum bit error ratio and corresponding optimum threshold.
One drawback of the above methods is that, since a minimum in the bit error ratio must be found, a search algorithm must be used, which requires a microprocessor controlled monitoring system running a complex program. Furthermore, the methods are not very adaptive since a change in BER can mean two things: either the minimum BER or the optimum threshold has changed. For instance, assume that the optical power changes and the input level to the decision gate is compensated for by an AGC amplifier. In this case, both the ones' and zeros' noise change by about the same factor, thereby raising the minimum BER, but the optimum threshold stays the same. This condition might occur when one or more channels in a WDM system is added or drops out. An example of V.sub.OPT changing while the minimum BER does not occurs when the gain of the amplifier preceding the decision gate changes. The optimum threshold shifts to account for the change in ones and zeros levels, but the signal-to-noise ratio, and hence the minimum BER, have not changed. Hence, in the above cases, an ambiguity results when a change in BER is detected. Therefore, a continuous hunting of V.sub.TH for the minimum BER is needed. This ambiguity in V.sub.OPT tends to reduce the overall receiver sensitivity.